It is known that mono-1-olefins (α-olefins), including ethylene, can be polymerized with catalyst compositions employing titanium, zirconium, vanadium, chromium, or other metals, often combined with a solid oxide and in the presence of cocatalysts. These catalyst compositions can be useful for both homopolymerization of ethylene, as well as copolymerization of ethylene with comonomers such as propylene, 1-butene, 1-hexene, or other higher α-olefins. Therefore, there exists a constant search to develop new olefin polymerization catalysts, catalyst activation processes, and methods of making and using catalysts that will provide enhanced catalytic activities and polymeric materials tailored to specific end uses.
Polyethylene (PE) produced by any number of methods generally contains small to moderate amounts of long chain branched molecules. In some instances, tong chain branching (LCB) is desired to improve bubble stability during film blowing or to enhance the processibility of resins prepared with metallocene catalysts. However for many uses, the presence of LCB is considered undesirable due to the increased elasticity that it typically imparts to the resins. Therefore the ability to control the LCB level in polyethylene using metallocene-based catalysts is a desirable goal.
One example of this need is seen in the use of bridged or ansa-metallocene catalysts, which are desirable catalysts for some purposes, but which may tend to produce polymer with LCB levels that are detrimental to film performance. Therefore, new catalyst compositions and methods that allow better control of LCB levels within a desired specification range is a desirable goal.